The present invention relates to a signal processing circuit for television signals and more particularly to a contour signal correction circuit for the television signal used in a television receiver receiving the television signal for interlaced scanning and converting the same to the signal for sequential (non-interlaced) scanning.
The interlaced scanning used in the NTSC television system eliminates a flicker phenomenon to be produced on the screen of the picture tube without requiring so wide a bandwidth for its television signals and without deterioruting the reproducibility of movement in the picture image so much.
But, due to the interlaced scanning, there is sometimes produced a scanning line flicker in the ruster on the Faceplate of the picture tube or a course striped structure of the scanned lines are observed, and thereby, the picture image reproduced on the Faceplate is deteriorated. It is well known that an image corresponding to the total number of the scanned lines is not reproduced in the human visual sensation but only that corresponding to 60-70% of the actual number of the scanned lines is sensed by the eyesight (refer, for example, to a report entitled "Screen and Scanning Arrangements for High Quality Television", Monthly Journal of NHK Technical Laboratory, Nov. 1981).
Against this, a device to avoid the above described deterioration in the picture image is known, which, in a television receiver, generates an auxiliary signal through production of an image signal between two adjacent scanning lines by interpolation and displays the image from the signal in between the adjacent scanning lines. For example, in the case of a circuit described in Japanese Paten Laid-open No. 58-77373/1983, movement of the picture image included in the video signal is detected, and the interpolation signal is produced from signals of a plurality of different fields when movement of the picture image is small, and the interpolation signal is produced from the signals within one field when movement of the picture image is large, and thus the circuit is enabled to make processing in a movement responsive manner. Through such processing, an interpolation signal to improve the resolution is generated for a still picture, while an interpolation signal which does not produce such deterioration in the picture image as blurring is generated for a moving picture, and thereby, an interlaced scanning signal is converted into a sequential scanning signal.
In a television receiver, there is in general provided a circuit for correcting the signal of the contour portion of the luminance signal. In the apparatus described in Japanese Patent Laid-open No. 60-19356/1985, for example, a composite color video signal for 2:1 interlaced scanning is converted into a digital signal, and then, the signal, in the first place, is converted into that for sequential scanning through a circuit for converting to sequential scanning. Separated from this digitized composite color video signal by means of a comb filter, a signal component including the vertical contour signal is obtained. And then, from the same, the chrominance signal component is removed by means of a low pass filter, and thus, the vertical contour signal is enabled to be obtained.
In the above described prior art, the received 2:1 interlaced scanning signal is first converted into a sequential scanning signal. While the frame frequency of the interlaced scanning signal is 30 Hz, the frame frequency of the sequential scanning signal is 60 Hz. It therefore follows that the circuit processing the sequential scanning signal must make the contour correction with the signal the operating speed therefor is doubled.
If the sampling frequency for converting the video signal to the digital signal is made to be four times the fsc (the fsc is the color subcarrier frequency of approximately 3.58 MHz), then this frequency becomes approximately 14 MHz, and therefore, the line memory to be used as the delay circuit and the contour detector circuit must be that functioning at approximately 28 MHz.